The present invention relates to a linear amplification with nonlinear components (LINC) system. More particularly, the invention relates to removing effects of gain and phase mismatch in such a LINC system without adjusting its analog circuits.
Many signal processing circuits have a need to maintain known relationships between the signals traversing multiple signal paths. In practice, however, imperfections within a circuit may affect such relationships and adversely impact the performance of the circuit. One example is that of an amplifier employing a linear amplification using nonlinear components (LINC) technique. Recently, this LINC technique has received much attention because the technique is suitable for wireless communication applications.
In a typical LINC amplifier, a signal to be amplified is decomposed into two signals, both having an equal and constant envelope. Baseband in-phase (I) and quadrature (Q) signal modulation of a radio frequency (RF) carrier signal may be used to obtain the two decomposed signals. The decomposed signals are separately amplified by two independent amplification paths with the same gain and delay (phase) then combined to form a linearly amplified signal. The LINC amplifier is attractive because the two decomposed signals to be amplified have a constant envelope that enables the use of efficient nonlinear radio frequency (RF) power amplifiers for amplification. Moreover, the final output may be highly linear and substantially free of intermodulationxe2x80x94a key consideration for bandwidth efficient wireless communications.
However, the promise of LINC amplifiers have yet to be fully realized in part because of the difficulty in developing circuits that maintain phase and gain balance for the two independent amplification paths. For a LINC transmitter, the gain and phase mismatch of the two amplification paths will not only affect its modulation accuracy but also degrade its out-of-band spectrum performance. The out-of-band spectrum performance of a transmitter is measured by the ratio of the maximum out-of-band signal level to the signal level inside the signal bandwidth. Decreasing this ratio results in a higher requirement on the matching condition. For example, it may be desirable to achieve an out-of-band spectrum performance of xe2x88x9265 dBc for adjacent channel coupled power, which, for a fairly simple modulation scheme, requires phase and gain match conditions within 0.5% for signals traversing the different amplification paths. Manufacturing imperfections within the amplification circuits make such balancing requirements difficult to achieve.
A variety of prior art approaches exists to address such circuit imperfections. In one approach, phase and power measurements are made of RF signals and such measurements used to adjust for signal path imbalance. In another prior art approach, a feedback loop is used which requires the sampling and processing of the RF signal to compare with the original baseband signal. This approach typically requires complex stability analysis to handle impedance load variations and other factors. Further, in prior art approaches, adjustments typically involve phase and amplitude corrections along the RF signal paths. However, any adjustment of the gain of an amplification path may also change the phase of the amplification path due to the well-known AM-PM phenomenon. Moreover, phase adjustment by impedance change may also affect the equivalent gain of the amplification path. Therefore, such adjustment processes involve the resulting signal being continuously calibrated and measured until a certain amount of accuracy is achieved. Moreover, gain adjustment to a non-linear amplifier in a deep saturate state is difficult since its output power level is insensitive to its input power level.
In one aspect, the invention includes a method for removing effects of gain and phase mismatch in amplification branches of a linear amplification using nonlinear components (LINC) system. The method includes receiving an input signal, calculating a relative phase and gain difference in the amplification branches, and generating appropriate phasing components. The input signal is then controllably separated into a plurality of branch signals of different but constant envelope. The mismatch between branches may cause each branch signal to have a different envelope. However, the phases of the branch signals are appropriately adjusted in a certain amount of corresponding phasing components, in consideration of mismatch effect, such that when the branch signals are recombined, the combined signal substantially replicates the input signal.
In another aspect, the invention includes a linear amplification using nonlinear components (LINC) system. The LINC system includes a phasing component generator, a plurality of phase modulators, a plurality of power amplifiers, and a combiner. The phasing component generator is configured to receive an input signal, and operates to control separation of the input signal into a plurality of branch signals by generating phasing components for the phase modulators after calculating a relative phase and gain difference in amplification branches. Each branch includes one phase modulator to phase modulate the branch signal. Each phasing component is then applied to a corresponding phase modulator in the branch. Each branch also includes at least one non-linear power amplifier to amplify the phase modulated branch signal. The magnitude of the resulting output branch signal is constant and modulation independent. The mismatch between branches may cause each branch signal to have a different envelope. The combiner combines the amplified branch signals of different but constant magnitudes such that when the branch signals from the amplification branches are recombined, the combined signal substantially replicates the input signal.